This invention relates generally to reflection of electromagnetic energy and in particular to a radar countermeasure utilizing electromagnetic energy reflection to confuse radar reception.
Modern air defense systems employ radar extensively for the detection and location of hostile aircraft. Radar is a system for detecting and locating targets by radiating electromagnetic energy and then receiving the electromagnetic energy reflected from the targets. In order to minimize the effectiveness of radar, attacking aircraft often disperse various materials which reflect electromagnetic energy and thereby confuse radar reception. A type of radar reflecting material which is often dispensed from airplanes for this purpose is commonly known as chaff or rope. The first material to be used as radar chaff is strands of metal foil, e.g. aluminum. Since these metallic strips act as dipoles which reflect the electromagnetic energy, their effectiveness requires that their length closely equal one half the wave length of the opposing radar. Several serious disadvantages are inherent with this requirement. The wave frequency bands at which radar is radiated are 8-18 and 26.5-40 GHz and hence the strand must be more than two inches long. Strands longer than two inches bend easily and tend to ball up or coil during free fall, thereby decreasing the effective radar reflection. Also by twisting up into a ball, the metal coils fall more quickly to the ground. In efforts to overcome this defect, great care and expense have been expended in packaging, transporting, and dispensing the strands of metal foil in order to prevent coiling and bending. The amount of success has been minimal because of the shock occurring at ejection into the air. Another problem arising from the strand's dependence on length is that several lengths must be dispensed although only one length is needed.
Another material often used for chaff is glass or plastic fibers coated with a relective material and formed into strands. The effectiveness of this type of chaff also depends on length and so its disadvantages are similar to strands of metal foil. While these reflective strands of plastic or glass do not bend or coil and are easier to pack and dispense, they do break during packing and especially upon impact with the air when they are dispersed.
Artificial ion clouds have also been used to reflect electromagnetic energy. The earlier techniques relied on explosions or thermal reactions to create the ion cloud. Although the use of ion clouds represented a significant advancement in the art of radar countermeasures, a number of problems still existed. Ionization was short lived because of diffusion, recombination, and attachment. Another disadvantage was the low percentage conversion of material into ions, which meant that the cost of an effective ion cloud was high.
Another method of producing an artificial ion cloud is to disperse particles of chemicals e.g. lithium hydride which photo-ionize. However the effectiveness of these chemicals is greatly affected by the humidity of the air. Also chemicals like lithium hydride are dangerous to handle, difficult to store, and are expensive.